Re: [PATCH] software node: only let the owner release a managed node

[Johan Hovold]

On Mon, Nov 09, 2026 at 03:10:31PM +0100, Danilo Krummrich wrote:
> On Mon Nov 9, 2026 at 2:37 PM CET, Johan Hovold wrote:
> > On Mon, Nov 02, 2026 at 04:25:16PM +0100, Bartosz Golaszewski wrote:
> >> device_create_managed_software_node() creates a node whose lifetime is
> >> tied to the device it was created for. software_node_notify_remove() is
> >> called for every device that has the node as its fwnode (primary OR
> >> secondary) and, if the node is managed, unconditionally detaches it from
> >> that device and drops the creation reference.
> >> 
> >> This breaks when the node is shared with another device. Example: the
> >> i2c-designware copies the parent's fwnode to the adapter's struct device.
> >> When the adapter is deleted, it releases the reference that belongs to
> >> the owner and the latter ends up with a dangling pointer.
> >> 
> >> Record the device's "manager" and only perform the actual release when
> >> notify is called for that "owner" device.
> >> 
> >> Reported-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@xxxxxxxxx>
> >> Closes: https://lore.kernel.org/all/20261009164241.923756-2-balasubramani.vivekanandan@xxxxxxxxx/
> >> Fixes: 151f6ff78cdf ("software node: Provide replacement for device_add_properties()")
> 
> Hm...the documentation of device_create_managed_software_node() does point out
> the limitation that it shouldn't be shared with other devices.
> 
> Now, that's rather fragile and the reason I pointed out [1] it should be
> changed, but it's technically not a fix?
> 
> [1] https://lore.kernel.org/all/DM1FMM3KEYU9.1D26IINII93FO@xxxxxxxxxx/

It's hard to determine at allocation time whether a node will eventually
end up being reused. That's why added the following clarification to the
commit message:

	Note that it was never the intention that managed software nodes
	would be shared, but descendants reusing ancestor firmware nodes
	is not uncommon and such use is hard to predict when allocating
	software nodes.
 
> >> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>

> > The end-result applied for 7.4 (to fix the 7.3 regression) is here:
> 
> A previous reply from Balasubramani suggested to adjust the doc-comment of
> device_create_managed_software_node(), so I thought Bartosz will send a new
> version.

I missed that comment (and his Reviewed-by tag, will add).

The function still does not return the allocated node and managed nodes
should generally not be shared. The exception would be descendants
reusing ancestor nodes, and that may happen elsewhere in the stack.

So I think the kernel doc is fine as-is, for now.

> Also, are we sure this is -rc material? I think the original commit wasn't
> technically broken and the bug in Xe was fixed otherwise.

The regression was introduced by 0f485f88656a ("drm/xe/i2c: use
device_create_managed_software_node()") in 7.3 and Bartosz framed it as
an alternative fix for that bug.

Are you saying this has since been fixed differently?

If so, this can obviously wait.

Johan